1. Field of the Invention
The present invention relates to an optical system having a polarization compensating optical system. More particularly, the present invention relates to a polarizing microscope or the like having a polarization compensating optical system capable of compensating for retardation with high accuracy.
2. Description of the Related Art
In optical apparatuses designed to observe even slight changes in the state of polarization, e.g. polarizing microscopes, it is important to hold the plane of polarization with high accuracy in the optical system. However, when light rays are incident on the lens surface of the optical system, the polarization plane may be disordered to a considerable extent, depending upon the incident angle and azimuth angle of the rays. Therefore, extinction cannot be achieved in a polarizing microscope, for example, even if it adopts the crossed Nicol arrangement, in which a polarizer and an analyzer are arranged such that their transmission axes are perpendicular to each other.
FIG. 30 schematically shows the optical system of a polarizing microscope. The optical system includes a light source 1, a collector lens 2, a polarizer 3, a condenser 4, a specimen 5, an objective 6, an analyzer 7, and an image-forming lens 8. Thus, an enlarged image for observation of the specimen 5 is formed. The axis of vibration of the polarizer 3 extends in the direction indicated by the arrow 10. The direction of the vibration axis of the analyzer 7 is as indicated by the arrow 11. Thus, the polarizer 3 and the analyzer 7 are arranged in crossed Nicol relation to each other. The pupil intensity distribution after the passage of light through the analyzer 7 is as shown in FIG. 31. Thus, not the entire pupil is shielded, but light leaks through peripheral portions of the pupil, as shown by white regions in the figure. Consequently, a dark cross-shaped pattern, which is called "isogyre", appears.
FIG. 32 shows a ray incident on the objective 6. The arrow 12 in the figure indicates the direction of the incident ray, and .theta. represents the azimuth angle of the incident ray. If the ray is linearly polarized light vibrating in the direction indicated by the arrow 13, the polarized light is decomposable into p-polarized light 14 and s-polarized light 15. In general, p-polarized light and s-polarized light are different in transmittance from each other. Therefore, after the polarized light ray has passed through the objective 6, the direction of vibration of the polarized light does not coincide with the direction 13. Accordingly, complete extinction cannot be achieved even if the crossed Nicol arrangement is adopted. Therefore, in the case of a polarizing microscope designed to observe even slight birefringence from the specimen 5, the observability of the microscope is restricted to a considerable extent by the disordered polarization plane.
A polarization compensating optical element called a "rectifier" is known as an optical element that suppresses the rotation of the polarization plane. The technique concerning the polarization compensating optical element is disclosed, for example, in Japanese Patent Post-Exam Publication No. 37-5782. According to the disclosed technique, an optical element of refracting power zero is inserted in front of the condenser or behind the objective, together with a .lambda./2 wave plate. The optical element has approximately the same polarizing characteristics as those of the condenser or the objective. In the case of a rectifier for the objective, for example, rays first pass through the objective, and the objective rotates the polarization plane of the rays. Then, the rays pass through the .lambda./2 wave plate. Consequently, the direction of vibration of polarized light changes to a direction symmetric with respect to the original axis of vibration (the axis of vibration of the polarizing plate). Thereafter, the rays pass through the optical element of refracting power zero. Consequently, the rotation of the polarization plane produced by the optical element cancels the reverse rotation of the polarization plane produced by the objective. Thus, the direction of the polarization plane of the rays emerging from the rectifier becomes approximately equal to that of the rays before entering the objective. Accordingly, the use of such a rectifier makes it possible to compensate for the rotation of the polarization plane produced by the objective or the condenser.
Japanese Patent Post-Exam Publication No. 52-37784 discloses another technique of suppressing the rotation of the polarization plane. According to the disclosed technique, coating is provided on an annular region of a lens within an area extending from 0.7 to 1 time the effective diameter of the lens. Parameters of the coating are calculated and optimized so that the difference in transmittance between p-polarized light and s-polarized light produced by the coating cancels the rotation of the polarization plane produced by the optical system to be compensated.
In general, objective and condenser lens systems are each provided with coating in order to increase the transmittance thereof. When rays are incident on the coated surface at an incident angle other than zero degree, the phase difference between the p- and s-polarized light components of the incident rays is changed by multiple interference in the coating. Therefore, when linearly polarized light whose p- and s-polarized light components are not zero is incident on the coated surface, for example, transmitted light rays generally become elliptically polarized light.
Thus, when light rays pass through such a coated optical system, the state of polarization is disordered by the above-described two causes: (a) the rotation of the polarization plane due to the transmittance difference between the p- and s-polarized light components; and (b) the phase difference (retardation) introduced between the p- and s-polarized light components by the coated surface.
In an objective or condenser whose numerical aperture is not so large, the amount of retardation produced therein is not large because the incident angle of light rays entering the lens surface is relatively small. Therefore, even if incident linearly polarized light is changed to elliptically polarized light by the retardation, the ellipticity is relatively small, and thus it can be regarded as substantially remaining linearly polarized. Accordingly, if the above-described rectifier is used to compensate for the disordered polarization plane, the cause (a) of disordering the state of polarization is eliminated. Thus, the use of the rectifier is effective to a certain extent.
However, in the case of an objective or condenser having a large numerical aperture, the incident angle of light rays entering the lens surface is large, and the amount of retardation produced therein increases. Further, such an objective or condenser uses an increased number of lens elements to correct aberrations in the whole lens system and has an increased number of coated surfaces to be passed by rays. Therefore, the retardation increases cumulatively. Consequently, the cause (b) of disordering the polarization plane becomes so large that it cannot be disregarded. Thus, it is difficult to compensate for the disordered state of polarization completely simply by eliminating the cause (a) of disordering the polarization plane using the polarization compensating optical element as disclosed in Japanese Patent Post-Exam Publication No. 37-5782 and so forth.
Thus, it is necessary to eliminate both the above-described causes (a) and (b) in order to compensate for the disordered state of polarization. One example of the technique of solving the problems is disclosed in U.S. Pat. No. 3,052,152. The disclosed technique uses the above-described rectifier to eliminate the rotation (a) of the polarization plane. Further, it cancels the retardation (b) by providing a phase difference such as to cancel the introduced retardation using a phase plate, e.g. a birefringent material. However, such a phase plate provides the same phase difference over the entire aperture of the lens system. A ray passing through each point in the aperture repeats incidence and refraction with various angles at the refracting surface of the lens. Therefore, rays passing through each point in the aperture have different amounts of retardation. Accordingly, this method is effective only for rays passing through a part of the aperture.
According to the above-described Japanese Patent Post-Exam Publication No. 52-37784, a multilayer coating consisting of at least two layers is provided on an annular region of a lens. The annular region lies within an area extending from 0.7 to 1 times the effective diameter of the lens. The multilayer coating is designed to suppress both rotation of the polarization plane and retardation as much as possible. Therefore, the coating design is very complicated.
In view of the above-described problems associated with the prior art, an object of the present invention is to provide a polarization compensating optical system capable of compensating for retardation with high accuracy. Another object of the present invention is to provide a polarization compensating optical system capable of favorably compensating for both rotation of the polarization plane and retardation, thereby improving the polarization performance to a considerable extent.